The present invention relates to thermal energy storage formulations. More particularly, the present invention relates to an improved water-based thermal energy storage formulation which disrupts ice structure and reduces volume expansion associated with the transformation of water to ice.
Phase change materials ("PCMs") store heat during phase transition, typically liquid/solid phase transitions. A large amount of thermal energy can be stored as latent heat of fusion during the melting of the PCM. For this reason, PCMs are often incorporated into thermal energy storage apparatuses. During the operation of such an apparatus, heat from the surrounding air is transferred to the PCM as heat of transition, until the frozen PCM completely melts. Additional heat from the surrounding air is then stored within the PCM as sensible heat. The heat stored within the PCM may be discharged from the apparatus by passing relatively cool air past the liquid PCM. The liquid PCM transfers its heat to the air stream, and thus, the temperature of the air stream is raised and the PCM is re-cooled.
Various attempts have been made to incorporate PCMs into heating and air conditioning systems, including heat pump systems, solar collection systems, and more conventional heating and air conditioning systems for homes, vehicles, and similar structures requiring heating and cooling. For example, U.S. Pat. No. 5,054,540 to Carr describes a cool storage reservoir positioned in an air duct of a vehicle on the like. Another example is the "heat battery" designed to provide "instant heating" to a vehicle cabin. (Automotive Engineering, Vol. 100, No. 2, February, 1992.)
A variety of materials may be used as PCMs. For example, water, paraffins, alcohols, and salt hydrates have notably high energy densities over temperature ranges of practical significance. Water, however, is of particular interest because it is plentiful, inexpensive, and environmentally friendly. As a heat storage material, it has good heat capacity, heat transfer properties, and an acceptable density. Additionally, the transformation of water into ice has a heat of fusion of 80 cal/g and occurs at 0.degree. C.
However, the use of water as a PCM presents some difficulties. For example, conventional air conditioning units must be reconfigured to operate at the ice temperature, and, in an air heat exchanger, moisture can freeze on the cooling coil. In addition to refrigeration considerations, the conversion of water to ice is accompanied by approximately a 9% volume expansion. This anomalous expansion during freezing is the cause of busted water pipes in homes during cold weather, and broken radiators/cracked engine blocks in cars. Moreover, if freezing water were to crack a thermal storage apparatus, causing liquid PCM to leak, a user relying on the PCM for climate control would be put at an unnecessary risk of exposure to extreme cold or heat. What is needed is a formulation for a PCM, which has the energy storage qualities of water but lacks the destructive freeze characteristics associated with water's transformation to ice.
According to the present invention, there is provided a method for reducing rupture failures during freezing cycles of a container device holding in a PCM composition comprising about 60 to 91 weight percent water by adding to the composition one or a mixture of non-ionic surfactants, preferably ethoxylates.
Also in accordance with the present invention there is provided a water-based thermal energy storage formulation capable of forming a frozen discontinuous water phase and of significantly maintaining the energy storage properties of water. The composition comprises about 60 to about 91 weight percent water and about 9 to about 40 weight percent of one or a mixture of water-dispersible non-ionic surfactants.
The project was undertaken to modify the freeze characteristics of water, specifically the single piece characteristic of ice and the considerable freeze expansion. The objective was to have ice form as discrete particles, forming a kind of "slippery ice" and/or to reduce the total expansion accompanying ice formation. The composition in accordance with the present invention prevents the formation of a single, solid, block of ice and reduces freeze expansion.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.